An ‘optimal' solution for the valuation of seagrass ecosystems

Background/Question/Methods

Seagrasses are primary producers who generally inhabit estuarine and marine environments in relatively shallow waters near the coastline. Seagrass ecosystems have been well-documented as nurseries for many marine animals, some of which are of significant commercial importance. They have also been shown to provide multiple ecosystem services including shoreline protection by reducing wave impact, improved water quality by holding sediment in place, and more recently have been explored as a significant sink for carbon dioxide. The shallow nature of this habitat and its proximity to human habitat has opened this system to extraordinary levels of anthropogenic impact. Conventional methods of assessing this impact have relied on the ‘existence’ value of seagrasses (WTP), as well as replacement estimates on the cost of restoration and prevention. WTP methods assumes total knowledge of the ecosystem by the responders and replacement costs are based on commodities that in actuality have no relationship to the specifics of the system. As a result, neither method explicitly incorporates ecological parameters and their relationship both with the species that reside in these systems, and their market value. We use optimal control theory to determine levels of sustainable harvest for pink shrimp (Farfantepenaeus duorarum) that is harvested in the Gulf of Mexico, but whose earlier life stages occur in Biscayne Bay, Florida. Optimal control theory is a differential model that calculates the required path for control variables to follow so that cost of a commodity or effort is minimized. We calculate these values based on the implicit assumption that habitat quality is the strongest determinant of stock size, and consequently the economic value of these systems can be calculated by quantifying the rate at which they can house residents. We used logistic regression to calculate growth constants for Gulf of Mexico shrimp stock, and the relative change in price with pounds caught. 

Results/Conclusions

For southern Biscayne Bay, we show that optimal shrimp harvest, considering the growth rate of seagrass patches, time needed for stock recovery, and cost of extraction, is 70,000 pounds of shrimp per year. At a n average market price of $1.71, this makes the annual contribution of shrimp at $119,700. This productivity method for estimating seagrass net value only considers its relationship with shrimp, and does not factor other commercially caught species, as well as the other estimable services that seagrass ecosystems provide.